The present invention relates to a linear actuator which is provided with a first level of position control and a second level of position and overload control.
Linear actuators are typically utilized in situations where a thrust force is used for applying linear motion. Examples of the utilization of such thrust force is in the operation of level arms, cranks, slides and valve levers in industrial equipment. Such actuators are utilized for alternatively moving objects between predetermined positional limits. The actuators can be utilized for moving the moveable member between positions within such predetermined limits by the utilization of appropriate feedback means.
Linear actuators usually include an electric drive motor which is connected to a drive screw through a drive means which utilizes either a gear train or a drive belt. Rotation of the drive screw by the electric motor causes a drive nut to move axially in order to impart axial motion to a connection extension rod. For unattended automatic operation of linear actuators it is necessary to provide a control means for disengaging the electric motor at the outer and inner limits of the extension rod stroke. Various arrangements many of which include microswitches have been utilized for this purpose.
U.S. Pat. No. 4,328,885 to Zouzoulas which is assigned to the common assignee hereof describes an actuator activated switching mechanism which provides for control of an electric motor depending upon the position of the actuator. Other control means for actuators are shown in U.S. Pat No. 2,441,505 to Ochtman; U.S. Pat. Nos. 3,209,090, 3,463,891, 3,480,746 and 3,504,555 all to Fry; and U.S. Pat. No. 3,529,104 to Angle.
For some applications it has been deemed preferable to employ an overload thrust disengagement control means so that blockage of the movement of the extendable member will not result in breakage of the actuator components. U.S. Pat. No. 4,307,799 to Zouzoulas provides a thrust control mechanism 41 for this purpose. Another type of overload control is the utilization of a slip clutch such as shown in U.S. Pat. No. 2,660,281 to Ochtman. These types of thrust control mechanisms and overload controllers are expensive to manufacture and often encounter internal breakage which then renders them unsuitable for their intended function of preventing further damage to the actuator.
Another feature of linear actuators is that various internally fitted rods or splines have been utilized to prevent rotation of specially designed drive nuts which are moved axially by rotation of the drive screws. U.S. Pat. No. 4,307,799 to Zouzoulas illustrates a drive nut 26 which is designed to be restrained against rotational motion by four guide rods 30. U.S. Pat. No. 4,137,784 to Griffin shows the use of an internal spline or protrusion 52 for preventing rotation of the drive nut 30. The use of such rods and splines or protrusions requires the special machining of drive nuts as well as the use of complicated internal components.
Another problem present in the prior art is that many of the linear actuator housings were not sealed against the incursion of dust in the industrial environments in which they are utilized. The absence of sealed housings then results in either the electric motor or the linear actuator extension rod becoming fouled and inoperative.